Define the data schema of a Web application for managing on-line auctions.

• Each auction can be opened or closed and concentrates on an object (the one offered in the auction).
• Each object is characterized by a name, a description, and a minimum bid, and can be presented by means of one or more pictures (with caption).
• Each object is also associated with all the bids raised during the auction, each bid beeing characterized by price and submission date.
• For each auction, data about the seller user and the bidding users must also be represented.
• In order to facilitate their retrieval, auction objects are grouped by category.